Candida albicans is the most common fungal pathogen, causing both mucosal and systemic infections, particularly in immunocompromised people. Until recently, this opportunistic pathogen was thought to be asexual, existing only as a diploid organism. However, a mating-type-like (MTL) locus was identified and genetic manipulation of it led to the laboratory construction of mating-competent strains of C. albicans. The products of mating are tetraploid strains that must undergo a reductional division to return to the diploid state if a complete sexual cycle is to be achieved. In C. albicans, this reductional division (from tetraploid to diploid) can occur by a non-meiotic mechanism involving concerted chromosome loss. The resultant diploids are themselves mating-competent, thereby completing a parasexual cycle in this organism. The goal of this study is to determine the mechanism of concerted chromosome loss in C. albicans, and to understand the role it plays in the life-cycle of the organism, with special emphasis on its possible role during infection. The pattern of chromosome loss from tetraploid strains will be mapped by taking intermediates in chromosome loss and hybridizing their DNA to Candida microarrays. This will reveal if certain chromosomes are lost preferentially, and whether they are lost in a prescribed order. Chromosome loss will also be examined by directly visualizing cells undergoing chromosome loss by microscopy. If chromosome non-disjunction is occurring then defects in the mitotic apparatus may be observed. The transcriptional profiles of diploid and tetraploid strains will also be compared. Particular emphasis will be placed on understanding the transcriptional program of tetraploid strains during chromosome loss. A major goal of these studies is to examine the role of the tetraploid and chromosome loss in infection. Therefore, the virulence of the tetraploid strain of C. albicans will be compared to that of diploid strains, and chromosome loss will be followed in models of infection. Finally, these studies will directly address whether Candida contains recessive lethal alleles in its diploid genome by construction of an allelic microarray. This will also allow recombination events that take place during the parasexual cycle of C. albicans to be identified.